This invention is concerned with a method for causing the isoparaffin-olefin alkylation reaction to occur in either a supercritical fluid or near-supercritical fluid phase over a solid catalyst at temperatures that are substantially lower than analogous supercritical fluid reactions previously reported for the reactants involved.
Alkylation is a reaction wherein an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with a C.sub.4 to C.sub.8 isoparaffin in the presence of an acidic catalyst producing a so-called alkylate. That alkylate is a valuable blending component in the manufacture of gasolines not only to increase its octane rating but also to increase its sensitivity to octane-enhancing additives.
The United States petroleum refining industry production of motor fuel alkylate was 425,700,000 barrels in 1996; with an anticipated growth of 25% over the next 5 years. The world wide motor fuel alkylate production was 812,500,000 barrels in 1996. (Michelle Williamson, "WORLDWIDE REFINING," Oil & Gas Journal, Special issue, 47-90, Dec. 18, 1995). The refining industry currently uses concentrated liquid acids, such as hydrofluoric acid and sulfuric acid, to catalyze the alkylation of aromatics and isoparaffins with olefins for the production of chemical commodities, intermediates, and gasoline blend stocks.
Paraffin alkylation, as discussed here, refers to the addition reaction of an isoparaffin and an olefin. The desired product is a higher molecular weight paraffin that exhibits a greater degree of branching. The principal industrial application of paraffin alkylation is in the production of premium-quality fuels for spark-ignition engines. Originally developed in the late 1930s to meet the fuel requirements of high performance aviation engines, alkylation is now primarily used to provide high octane blending components for automotive fuels. Future gasoline specifications will continue to require the clean burning characteristics and the low emissions typical of the product of paraffin alkylation.
Although alkylation of paraffins can be carried out thermally, catalytic alkylation is the basis of processes in commercial use. Early studies of catalytic alkylation led to the formulation of a proposed mechanism based on a chain of ionic reactions. In practice, however, the paraffin alkylation reaction is a complex reaction that cannot be explained solely by the chain mechanism. Historically, the catalysts used in the industrial paraffin alkylation reaction are strong liquid acids, either sulfuric acid or hydrofluoric acid. Other strong acids have been shown to be capable of promoting alkylation in the laboratory, but have not been used commercially.
Industrial alkylation processes have historically used a hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. The sulfuric acid alkylation reaction is particularly sensitive to temperature, with low temperatures being used to minimize the side reaction of olefin polymerization. Acid strength in those liquid acid catalyzed alkylation processes is preferably maintained at 88 to 94 weight percent acid by the continuous addition of fresh acid and the continuous withdrawal of spent acid. The hydrofluoric acid process is less temperature sensitive and the acid is easily recovered and purified.
Both sulfuric acid and hydrofluoric acid alkylation share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal problems. Indeed, the toxic nature of hydrofluoric acid has earned it a place on the "ultra hazardous" chemicals list of the EPA. Moreover, the waste sludge produced by either the sulfuric or hydrofluoric acid catalyzed reactions is classified as a hazardous waste and, therefore, is under the stringent controls and regulations of RCRA and CERCLA regulation regimes. The additional expense that such regulated waste management can cost a manufacturer is significant. Research efforts have therefore been directed to developing alkylation catalysts that are as effective as sulfuric or hydrofluoric acids but lack many of the problems associated with those two acids. For a general discussion of acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
With the increasing demands for octane and the increasing environmental concerns, it has been desirable to develop an alkylate process using safer, more environmentally acceptable catalyst systems. Specifically, it is desirable to provide an industrially viable alternative to the currently used hydrofluoric and sulfuric acid catalyzed alkylation processes. Consequently, substantial efforts have been directed to developing a viable isoparaffin-olefin alkylation process that avoids the environmental and safety problems associated with sulfuric and hydrofluoric acid alkylation systems while retaining the alkylate quality and reliability characteristic of those well-known processes. Research efforts have been directed toward solid as well as liquid alkylation catalyst systems, as reflected in the following references.
U.S. Pat. No. 5,304,698 (hereinafter, "the '698 patent") delineates the three requirements for the alkylation reaction to occur. The reaction must have feedstocks, a suitable catalyst for promoting the reaction, and reaction conditions that promote and sustain the reaction. The general method taught for olefin conversion in the catalytic alkylation of an olefin with an isoparaffin comprises contacting an olefin-containing feed with an isoparaffin-containing feed with a microporous zeolite material. Such zeolite-containing catalysts useful in the process disclosed in the '698 patent include ZSM-4, ZSM-12, ZSM-20, ZSM-35, ZSM-48, ZSM-50, MCM-22, PSH-3, TMA offretite, TEA mordenite, clinoptilolite, mordentite, REY, and zeolite Beta. Further teaching regarding those solid catalysts and their formation can be found in U.S. Pat. Nos. 4,439,409; 3,832,449; 3,972,983 (ZSM-20); 4,016,245 (ZSM-35); 4,397,827 (ZSM-48); 4,640,849 (ZSM-50); 3,308,069 (Beta); and 4,439,409 (PSH-3). The '698 patent further teaches that the group of useful catalysts includes porous crystalline solids and layered materials. The non-zeolitic inorganic oxide of the solid catalyst may be selected from the group of diverse inorganic oxides, such as alumina, zirconium dioxide, chromia, zinc oxide, magnesia, calcium oxide, silica-alumina, silica-magnesia, silica-alumina-magnesia, and silica-alumina-zirconia, as well as the naturally occurring inorganic oxides of various states of purity such as bauxite, clay and diatomaceous earth.
Feedstocks useful in practicing the invention disclosed in the '698 patent include at least one isoparaffin and at least one olefin. The isoparaffin contains from about 4 to about 8 carbon atoms. The olefin component of the feedstock includes at least one olefin having from 2 to 12 carbon atoms.
The reaction conditions disclosed in the '698 patent include temperatures that are at least equal to the critical temperature (T.sub.c) of the principal component of the feed and pressures at least equal to the critical pressure (P.sub.c) of the principal component of the feed. The term "principal component", as used in the '698 patent, is defined as the component of highest concentration in the feedstock. Specifically, in a preferred embodiment of the '698 patent, the fresh crystalline microporous material contacts the mixed isoparaffin-olefin feed under process conditions that are at least equal to the critical temperature and pressure of the principal component of the feed. The alkylation process disclosed in the '698 patent is suitably conducted at temperatures from about 135.degree. C. up to about 371.degree. C., preferably from about 149.degree. C. to about 316.degree. C. The '698 patent also specifies that the operating temperature must be at least equal to the critical temperature of the principal component in the feed; while simultaneously the pressure must be maintained at least equal to the critical pressure of the principal component in the feed. Finally, the '698 patent teaches the use of conditions under which the principal reaction component, being in a supercritical fluid state, prolongs the useful catalytic life of the crystalline microporous material through physical/chemical mechanisms and properties attributed to the supercritical phase of matter.
U.S. Pat. No. 5,345,028 (hereinafter, "the '028 patent") discloses several methods for the preparation and use of solid catalysts in the olefin isoparaffin alkalation reaction. The '028 patent also discloses three useful methods for the synthesis of solid catalysts of the type used in the present invention. The first disclosed method involves the formation of a sulfated Ni--Ti composition through the addition of NiCl.sub.2 to a solution of TiCl.sub.4 The resultant Ni--Ti composition is sulfated through the addition of sulfuric acid, and the resultant product is dried and activated according to methods well known in the art. An additional method involves the addition of Ti(OH).sub.4 to Ni(NO.sub.3).sub.2 in an aqueous environment. Upon the complete dissolution of the NiCl.sub.2 the composition is dried and activated according to methods well known in the art. The final method of catalyst formation disclosed in the '028 patent involves the addition of freshly prepared TiO.sub.2 to an aqueous solution of Ni(NO.sub.3).sub.2. The mixture is stirred, isolated, dried and activated according to methods well known in the art.
The two-part articles "Modern Alkylation", by Lyle F. Albright, published in the Nov. 12 and 26, 1990, issues of the Oil and Gas Journal summarizes the present sulfuric acid and hydrogen fluoride acid alkylation technology.
Thus, while it would be desirable to substitute a solid alkylation catalyst for the liquid catalysts described above, solid catalysts have not proven in the past to be a commercially viable alternative to liquid acid catalysts due to problems with catalyst longevity and alkylate product quality. Additionally, where some success involving the use of solid alkylation catalysts has been published, the reaction conditions require operating at relatively high temperatures. Notably those temperatures are required to be at or above the critical point of the reaction mixture. As previously reported, catalyst deactivation is attenuated and catalyst life is increased when reaction conditions are at or above the critical point of the reaction mixture. However, at those higher temperatures undesirable side reactions, including product isomerization, product cracking, olefin oligomerization and coking, predominate over the desired alkylation reaction, significantly reducing product quality and high octane product yield.